1. Field of the Invention
The present invention relates to a method of correcting color conversion data for converting color image data into device data in an image output device thereby to achieve a desired color reproduction depending on different color reproduction accuracies, color reproduction status variations, color perception differences due to different observational environments, etc.
2. Description of the Related Art
Image output devices for outputting a color image to an output medium such as a printed document, a hard copy, a CRT, etc. reproduce the color image by achieving area and density modulations of a halftone dot percentage on the output medium based on device data of colors C, M, Y, K or R, G, B. The reproduced color image has color characteristics largely depending on the properties of color materials and a support medium such as an output sheet of paper, fluorescent materials, etc.
There have been proposed a variety of processes for determining device data capable of obtaining desired color images with respect to various many image output devices and output mediums. For example, Japanese laid-open patent publication No. 3-131920 discloses a process of determining device data by a direct mapping conversion using a predetermined color conversion table produced for a certain printer or under certain printing conditions. According to another process revealed in Japanese laid-open patent publication No. 4-362869, a maximum likelihood solution of a coefficient for a masking process is determined to equalize calorimetric values of different image output devices, and device data are obtained using the coefficient.
However, even when a color image is reproduced with color conversion data processed for highly accurate color matching, the color image may not have colors that are sufficiently satisfactory.
Specifically, color images reproduced by image output devices of the same type which operate under the same output principles and output conditions may suffer color errors even if color conversion data capable of equalizing colors metamerically with a target color are established for the image output devices because the individual image output devices are subject to characteristic differences and aging.
When a proof for a color image is produced, a target output medium such as a printed document, a hard copy, a CRT, etc. and an output medium used as the proof may not necessarily have the same spectral characteristics as each other. If an observational light source has a different spectral distribution, then colors reproduced on the proof differ from a target color.
Furthermore, since color conversion data established for image output devices are determined using color matching functions based on average human visual characteristics, reproduced colors may be perceived differently by different human observers due to their visual perception differences.
One image output device for simulating finished properties of a printed document on a CRT is disclosed in Japanese laid-open patent publication No. 60-114862, for example. For correcting color conversion data of individual image output devices to equalize reproduced colors as they are visually perceived by human observers in view of different image output device characteristics and observational conditions, outputted results on output mediums are compared with each other based on colors simulated by the disclosed image output device until the colors as visually perceived are equalized on a trial-and-error basis. However, such a process needs a considerable amount of labor and skill for adjusting output colors.
It is quite difficult to correct only a certain color in view of different image output devices and different observational conditions and human observers while saving color conversion data of other colors. For example, Japanese laid-open patent publications Nos. 4-196676, 4-196678, and 4-217167 disclose processes for correcting the saturation, lightness, or hue of each of the colors in an isochromatic difference space such as an L*a*b* color space, and these disclosed processes require a large amount of corrective calculations because they are carried out three-dimensionally. Such a large amount of corrective calculations tends to disrupt an overall color tone and a continuity of gradations. Specifically, inasmuch as control quantities for actual colors of C, M, Y, K or R, G, B that are to be corrected do not monotonously increase (decrease) as control quantities in the isochromatic difference space monotonously increase (decrease), when device data are determined and outputted using data corrected in the isochromatic difference space, a discontinuous mapping relationship is likely to appear.